. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, by a Concave Mirror.—To find thepositi(Mi of an image as formed by a concave mirror, tworays may be used : one drawn from a given point on theobject to the mirror, and parallel to its principal axis, andreflected through the principal focus (, Pigs. 5 and 6);the other, the secondary axis, from the same point, passing OPTICS. 15 through the center of curvature. The place where thesecondary axis and the reflected ray or their projections in-tersect gives the position
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, by a Concave Mirror.—To find thepositi(Mi of an image as formed by a concave mirror, tworays may be used : one drawn from a given point on theobject to the mirror, and parallel to its principal axis, andreflected through the principal focus (, Pigs. 5 and 6);the other, the secondary axis, from the same point, passing OPTICS. 15 through the center of curvature. The place where thesecondary axis and the reflected ray or their projections in-tersect gives the position of the image. Unhke the planemirror, which produces images at all times and at all dis-tances, the concave mirror produces either an erect, virtual,and enlarged image, as an object is placed closer than itsprincipal focus, or an enlarged inverted image if the objectis between the principal focus and the center of curvature. By withdrawing the mirror in the former instance theerect image increases slightly in size, and in the latter theinverted image diminishes in size. At the principal focusthere is no imacfe Fig. 5. Figure 5 shows an erect, virtual, and enlarged image ofA R which is closer to the mirror than the principal rays from A and R are reflected to the principalfocus, Lines drawn from the center of curvaturethrough A and R to the mirror are secondary axes ; theselines and those reflected to the principal focus do not inter-sect in front of the mirror, but if projected, will meet at aand ;- behind the mirror, forming a magnified image ofA R. If the mirror is withdrawn from the object, theerect magnified imag-e would increase in size, but at theprincipal focus no image would be formed, as the rayswould be reflected parallel. i6 REFRACTION AND HOW TO REFRACT. Figure 6 shows a real inverted image of A R at ^ r;A R situated beyond the principal focus. Lines drawnfrom A and R through are secondary axes. Parallelrays from A and R converge a
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